Chloroplasts evolved following an endosymbiotic event between an ancestral, photosynthetic cyanobacterium and an early eukarytoic Phagotroph.[9] This event (termed Primary endosymbiosis) resulted in the origin of the red and Green algae, and the Glaucophytes, which make up the oldest evolutionary lineages of photosynthetic eukaryotes.[10] In the red algae, a secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages.[10]

Unicellular members of the Cyanidiophyceae are thermoacidophiles and are found in sulphuric hot springs and other acidic environments.[12] The remaining taxa are found in marine and freshwater environments. Most rhodophytes are marine with a worldwide distribution, and are often found at greater depths compared to other seaweeds because of dominance in certain pigments (i.e., Phycoerythrin) within their chloroplasts.[13] Some marine species are found on sandy shores, while most others can be found attached to rocky substrata.[13] Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats;[5] they generally prefer clean, high-flow streams with clear waters and rocky bottoms, but with some exceptions.[14] A few freshwater species are found in black waters with sandy bottoms [15] and even fewer are found in more lentic waters.[16] Both marine and freshwater taxa are represented by free-living macroalgal forms and smaller endo/epiphytic/zoic forms, meaning they live in or on other algae, plants, and animals [6] In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts [17][18]

One of the oldest fossils identified as a red alga is also the oldest fossil eukaryote that belongs to a specific modern taxon. Bangiomorpha pubescens, a multicellular fossil from arctic Canada, strongly resembles the modern red alga Bangia despite occurring in rocks dating to 1.2 billion years ago.[1]

Red algae are important builders of limestone reefs. The earliest such coralline algae, the solenopores, are known from the Cambrian period. Other algae of different origins filled a similar role in the late Paleozoic, and in more recent reefs.

In the system of Adl et al. 2005, the red algae are classified in the Archaeplastida, along with the glaucophytes and green algae plus land plants (Viridiplantae or Chloroplastida). The authors use a hierarchical arrangement where the clade names do not signify rank; the class name Rhodophyceae is used for the red algae. No subdivisions are given; the authors say, "Traditional subgroups are artiﬁcial constructs, and no longer valid."[21]

Many studies published since Adl et al. 2005 have provided evidence that is in agreement for monophyly in the Archaeplastida (including red algae).[22][23][24][25] However, other studies have suggested Archaeplastida is paraphyletic.[26][27] As of January 2011[update], the situation appears unresolved.

Below are other published taxonomies of the red algae using molecular and traditional alpha taxonomic data; however, the taxonomy of the red algae is still in a state of flux (with classification above the level of order having received little scientific attention for most of the 20th century).[28]

If one defines the kingdom Plantae to mean the Archaeplastida, the red algae will be part of that kingdom

If Plantae are defined more narrowly, to be the Viridiplantae, then the red algae might be considered their own kingdom, or part of the kingdom Protista.

Over 7,000 species are currently described for the red algae,[3] but the taxonomy is in constant flux with new species described each year.[28][29] The vast majority of these are marine with about 200 that live only in fresh water.

Chromalveolates seem to have evolved from bikonts that have acquired red algae as endosymbionts. According to this theory, over time these bikonts and their endosymbiont red algae have evolved to become chromalveolates and their chloroplasts. This part of endosymbiotic theory is supported by various structural and genetic similarities.[37]

The δ13C values of red algae reflect their lifestyles. The largest difference results from their photosynthetic metabolic pathway: algae that use HCO3 as a carbon source have far more negative δ13C values than those that only use CO2.[38] An additional difference of about 1.71‰ separates groups intertidal from those below the lowest tide line, which are never exposed to atmospheric carbon. The latter group uses the more 13C-negative CO2 dissolved in sea water, whereas those with access to atmospheric carbon reflect the more positive signature of this reserve.

Red algae are red due to phycoerythrin. They contain the sulfated polysaccharide carrageenan in the amorphous sections of their cell walls, although red algae from the genus Porphyra contain porphyran. They also produce a specific type of tannin called phlorotannins, but in lower amount than brown algae do.

Red algae have double cell walls.[39] The outer layers contain the polysaccharides agarose and agaropectin that can be extracted from the cell walls by boiling as agar.[39] The internal walls are mostly cellulose.[39]

Pit connections and pit plugs are unique and distinctive features of red algae that form during the process of cytokinesis following mitosis.[40][41] In red algae, cytokinesis is incomplete. Typically, a small pore is left in the middle of the newly formed partition. The pit connection is formed where the daughter cells remain in contact.

Shortly after the pit connection is formed, cytoplasmic continuity is blocked by the generation of a pit plug, which is deposited in the wall gap that connects the cells.

Connections between cells having a common parent cell are called primary pit connections. Because apical growth is the norm in red algae, most cells have two primary pit connections, one to each adjacent cell.

Connections that exist between cells not sharing a common parent cell are labeled secondary pit connections. These connections are formed when an unequal cell division produced a nucleated daughter cell that then fuses to an adjacent cell. Patterns of secondary pit connections can be seen in the order Ceramiales.[41]

After a pit connection is formed, tubular membranes appear. A granular protein, called the plug core, then forms around the membranes. The tubular membranes eventually disappear. While some orders of red algae simply have a plug core, others have an associated membrane at each side of the protein mass, called cap membranes. The pit plug continues to exist between the cells until one of the cells dies. When this happens, the living cell produces a layer of wall material that seals off the plug.

The pit connections have been suggested to function as structural reinforcement, or as avenues for cell-to-cell communication and transport in red algae, however little data supports this hypothesis.[42]

The trichogyne will continue to grow until it encounters a spermatium; once it has been fertilized, the cell wall at its base progressively thickens, separating it from the rest of the carpogonium at its base.[2]

Upon their collision, the walls of the spermatium and carpogonium dissolve. The male nucleus divides and moves into the carpogonium; one half of the nucleus merges with the carpogonium's nucleus.[2]

Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free-living) tetrasporophyte phase.[43] Tetrasporangia may be arranged in a row (zonate), in a cross (cruciate), or in a tetrad.[2]

The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp.[43]

These case studies may be helpful to understand some of the life histories algae may display:

In the Carposporophyte: a spermatium merges with a trichogyne (a long hair on the female sexual organ), which then divides to form carposporangia – which produce carpospores.

Carpospores germinate into gametophytes, which produce sporophytes. Both of these are very similar; they produce monospores from monosporangia "just below a cross wall in a filament"[2] and their spores are "liberated through apex of sporangial cell."[2]

The spores of a sporophyte produce either tetrasporophytes. Monospores produced by this phase germinate immediately, with no resting phase, to form an identical copy of parent. Tetrasporophytes may also produce a carpospore, which germinates to form another tetrasporophyte.[verification needed][2]

The gametophyte may replicate using monospores, but produces sperm in spermatangia, and "eggs"(?) in carpogonium.[2]

In its diploid phase, a carpospore can germinate to form a filamentous "conchocelis stage", which can also self-replicate using monospores. The conchocelis stage eventually produces conchosporangia. The resulting conchospore germinates to form a tiny prothallus with rhizoids, which develops to a cm-scale leafy thallus. This too can reproduce via monospores, which are produced inside the thallus itself.[2] They can also reproduce via spermatia, produced internally, which are released to meet a prospective carpogonium in its conceptacle.[2]

Several species are important food crops, in particular members of the genus Porphyra, variously known as nori (Japan), gim (Korea), or laver (Britain). Dulse (Palmaria palmata)[44] is another important British species.[45] These rhodophyte foods are high in vitamins and protein and are easily grown; for example, nori cultivation in Japan goes back more than three centuries.

In East and Southeast Asia, agar is most commonly produced from Gelidium amansii.